\documentclass{article}
\usepackage{booktabs,makecell,multirow,tabularx}
\renewcommand\theadfont{\bfseries\normalsize}
\newcolumntype{C}{>{\centering\arraybackslash}X}

\usepackage{geometry}

\begin{document}
    \begin{table}[ht]
\centering
\begin{tabularx}{\textwidth}{c L c L c L c }
    \toprule
\multirow{2}{*}{\thead{Atlas\\ Resolution}}
    &   \multicolumn{2}{c}{\thead{Variant-1}}
        &   \multicolumn{2}{c}{\thead{Variant-2}}
            &   \multicolumn{2}{c}{\thead{Variant-3}}    \\
    \cmidrule(lr){2-3}\cmidrule(lr){4-5}\cmidrule(lr){6-7}
    &   \thead[b]{Average\\ Render\\ Time (ms)}  &   \thead[b]{IFPS}
        &   \thead[b]{Average\\ Render\\ Time (ms)}  &   \thead[b]{IFPS}
            &   \thead[b]{Average\\ Render\\ Time (ms)}  &   \thead[b]{IFPS}                \\
    \midrule
512$\times$512      &  &  &  &  &  &  \\
1024$\times$1024    &  &  &  &  &  &  \\
    \addlinespace[2pt]
18432$\times$18432  &  &  &  &  &  &  \\
19456$\times$19456  &  &  &  &  &  &  \\
    \bottomrule
\end{tabularx}
    \caption{List of sizes for various resolution of Dataset-I, compressed variant Dataset-II and different slices variant Dataset-II}
    \label{tab:1}
\end{table}
\end{document}

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